The Callanish Stones or Callendish Standing Stones were constructed between 2900 and 2600 BC. The first written reference to the stones was by Lewis native John Morisone, who in c. 1680 wrote that "great stones standing up in ranks [...] were sett up in place for devotione". It has been speculated, among other theories, that the stones form a calendar system based on the position of the moon. Professor Alexander Thom suggested that the alignment of the stone avenue (when looking southward) pointed to the setting of midsummer full moon behind a distant mountain called Clisham. The tallest of the stones marks the entrance to a burial cairn where human remains have been discovered. An excavation campaign in 1980 and 1981 showed that the burial chamber was a late addition to the site, and that it had been modified a number of times.